A known current mirror circuit arrangement includes an output transistor whose collector electrode provides an output current and whose emitter electrode is connected to a reference potential via a current control resistor.
Connected between the base electrode of the output transistor and the reference potential is a series combination of a resistor and a diode, the diode usually taking the form of a transistor whose base and collector electrodes are directly connected together.
The above known circuit is conveniently driven from a current source which provides an input current and in this case the input and output currents are directly proportional to one another. That is to say any change in the input current fed to the current mirror circuit produces a substantially proportional change in the output current provided by the output transistor.
This linear relationship applies over a wide range of input and output currents provided that the output transistor has a satisfactory value of current gain and that the matching of emitter areas of the diode and output transistors is good. Also provided that the resistors connected in the emitter circuit of the output transistor and to the diode respectively are of equal value, the output and input currents will be equal to one another.
Although the relationship between input and output currents in the above circuit is linear the actual input impedance is not linear with respect to the input current.
The input impedance depends upon the sum of the values of the impedances of the fixed resistor and the diode connected between the base electrode of the output transistor and the reference potential.
The value of the internal impedance of the diode is dependent upon the magnitude of the current flowing through the diode. The greater the magnitude of the current through the diode the smaller is its internal impedance.
A problem arises with the above known current mirror circuit arrangement in that it is often not convenient to drive the circuit from a current source. This would usually be the case when the circuit is used as part of a television video circuit.
In this case the circuit would be driven either from a voltage source or, as is more likely, from a non-ideal current source formed by a voltage source and a series resistor.
In the former case in which a voltage source is used the series-connected resistor and diode would have no effect on the input voltage/output current characteristic. This would be dependent upon the internal emitter impedance of the output transistor and since, like that of the diode, this impedance varies non-linearly with current, becoming smaller at higher current values, this characteristic would be non-linear at lower currents only approaching linearity at high current values.
Similarly in the second case of a non ideal current source the input voltage/output current characteristic is non linear this non-linearity depending upon the relationship of the total fixed resistance, formed by the sum of the series resistance connected to the voltage source, referred to as the source resistance, and the resistance connected in series with the diode, to the value of the variable component formed by the internal impedance of the diode.
Since the variable component becomes more negligible at higher-currents a known solution to the problem of non-linearity is to drive the current mirror circuit at high current levels. This solution is, however unsatisfactory since operation at high currents is wasteful of current and increases thermal dissipation. Consequently, specifications for integrated circuits utilising current mirror circuits increasingly require the use of relatively low currents at which the non-linearity problem would occur yet still require a linear input voltage/output current characteristic.
Further, integrated circuits frequently operate from relatively low voltage power supplies and the use of high currents under these circumstances can cause problems due to the d-c voltage across circuit resistors.